1. Field of the Invention
The present invention relates to an apparatus for producing an optical fiber porous glass preform. More specifically, it relates to a technique for realizing stable production of an optical fiber porous glass preform (an optical fiber soot preform) by depositing glass particles generated in a flame by the VAD (vaporized axial deposition type CVD) method on a target (seed bar), for improving the efficiency of production of an optical fiber porous glass preform, and for reducing the height of the apparatus for producing the optical fiber porous glass preform.
2. Description of the Related Art
A variety of optical fibers are known. For example, a single mode optical fiber consists of a core of 10 .mu.m diameter in the center, a cladding of 125 .mu.m diameter surrounding the core, and a protective jacket formed by resin covering the cladding.
The method of production of such an optical fiber will be briefly described for background purposes. The main part of the optical fiber, that is, the core and the cladding, is formed by heating and drawing an optical fiber glass preform. The optical fiber glass preform is consequently also comprised of a core part and a cladding part. This optical fiber glass preform itself is formed by converting a soot body for forming the optical fiber porous glass preform into transparent glass.
As typical methods of production of an optical fiber porous glass preform, the OVD method (outer deposition type CVD method) and the VAD method are known. The present invention specifically relates to a method of production of an optical fiber porous glass preform using the VAD method.
Next, the general method of production of an optical fiber porous glass preform using the VAD method will be described.
(1) A seed bar (hereinafter referred to as a "target bar") is first prepared. This target bar is placed inside a reaction container (a reaction chamber) with one end suspended from an upper side so that the target bar can be rotated around its longitudinal center axis.
(2) Oxygen, hydrogen, and other combustion gases and the SiCl.sub.4 glass particle material (including a dopant such as GeCl.sub.4 if needed) are fed to oxyhydrogen burners from which oxyhydrogen flames are generated. In the oxyhydrogen flames formed by the combustion gases from the burners, the moisture in the oxyhydrogen flames and the SiCl.sub.4 undergo a hydrolysis reaction as shown by the following reaction formula and form SiO.sub.2, which is the main component of the glass particles. EQU SiCl.sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +4HCl
(3) These glass particles are sprayed to the lower part of the rotating target bar and deposited thereon to form the optical fiber soot body.
As described above, the optical fiber soot body formed by the VAD method is then converted to transparent glass to form the optical fiber porous glass preform used for producing an optical fiber. Note that an optical fiber soot body converted to transparent glass can further have glass particles deposited around it, if necessary. In this case, after depositing the glass particles, the soot body is again converted to transparent glass to form the optical fiber glass preform.
For wide application of the optical fiber produced by the above method, it is desirable that a high quality optical fiber with low transmission loss can be produced and that the optical fiber porous glass preform and, in turn, the optical fiber can be produced efficiently at low cost. A variety of proposals have been made concerning this so far.
In the related art, when synthesizing an optical fiber soot body with a small diameter of about 100 mm using the VAD method, since the burners need only produce a small heating power, there is almost no interference between the core burner and the cladding burner. Further, not enough glass particles are deposited on the inner wall of the reaction chamber to cause any problems.
In recent years, in an attempt to produce an optical fiber effectively at low cost, the optical fiber porous glass preform has been increased in size. Especially, due to the recent advances made in automatic conveyance techniques, the production of a large sized optical fiber porous glass preform of more than 10 kg, which had been difficult to move by hand, has been achieved. To produce an optical fiber soot body with a diameter of more than 200 mm, it is necessary to increase the heating power of the burners and to increase amount of source gases fed to the burners. However, when setting conditions to prevent disturbances in the flow of glass particles toward the optical fiber soot body, there is the disadvantage that glass particles will deposit on the inner wall of the reaction chamber.
Various methods have been proposed to overcome the disadvantage. The following are some known examples of production of a large sized optical fiber soot body.
Japanese Unexamined Patent Publication (Kokai) No. 62-171939 discloses an apparatus for producing an optical fiber porous glass preform (porous optical fiber preform) by the VAD method which eliminates the fluctuations in the flame so as to produce an optical fiber porous glass preform having a predetermined distribution of composition on a target bar (starting bar).
Describing in more detail the apparatus for producing an optical fiber porous glass preform, that is, an optical fiber soot body, disclosed in the above Japanese Unexamined Patent Publication (Kokai) No. 62-171939, this apparatus comprises a reaction chamber, a means connected to the reaction chamber for generating a gas flow inside the reaction chamber, burners, a means for stabilizing the flames of the burners by supplying air or inert gas such as nitrogen gas through a filter provided on the side of the reaction chamber, and a duct provided on the reaction chamber at the side opposite to the filter. The target bar is suspended from the upper side of the reaction chamber and rotated. Glass particles emitted from the burners are deposited on the tip of the lower part of the target bar. That is, the burners are arranged at a lower position in the reaction chamber from where they can eject glass particles to the tip of the lower part of the target bar positioned above the same.
In this apparatus, as the optical fiber soot body grows larger due to the deposition and accumulation of the glass particles on the tip of the lower part of the target bar, the target bar is pulled upward in the reaction chamber. Accordingly, a mechanism which pulls the target bar upward while rotating it is provided at the upper part of the reaction chamber.
However, the apparatus for producing an optical fiber porous glass preform disclosed in the above Japanese Unexamined Patent Publication (Kokai) No. 62-171939 suffers from the following disadvantages:
1. While the intention is to provide a uniform flow of air from a direction perpendicular to the target bar by using the filter and to thereby suppress fluctuation in the flames from the burners to the target bar to stabilize the same, when carrying out experiments with this kind of apparatus, it was found that due in part to the tapered shape of a duct, the glass particles which did not deposit on the target bar ended up adhering to the top surface of the duct resulting in insufficient exhaust. When the air flow through the filter was increased to achieve sufficient exhaust, there was a large fluctuation in the flames.
2. The flows of air in the lateral direction at the upper, center, and lower portions of the reaction chamber end up differing in speed. As a result, it was difficult to obtain stable deposition (growth) of the optical fiber soot body.
3. The reaction chamber of this apparatus was large in structure including, as it did, the part for pulling up the target bar. Along with the deposition of glass particles, the target bar was pulled upward. This structure of a reaction chamber is suitable when the size of the optical fiber soot body is small, however, when pulling up a large optical fiber soot body, it is necessary to provide a chuck with a longer stroke to support the target bar from the top and is necessary to provide a through hole at the upper part of the reaction chamber.
4. Along with the growth of the optical fiber soot body at the upper part of the reaction chamber, the flow of air concentrates at the upper part of the reaction chamber. As a result, the glass particles adhered to the inner wall of the reaction chamber are detached and float freely. Some of the floating glass particles adhere to the optical fiber soot body resulting in air bubbles in the body later on. This may cause a deterioration in the quality of the optical fiber soot body.
Japanese Unexamined Patent Publication (Kokai) No. 1-242431 discloses an apparatus for producing an optical fiber porous glass preform (glass particle deposition apparatus) using the VAD method etc. where a fan is provided to forcibly send air to an exhaust outlet of the reaction chamber surrounding the optical fiber soot body (portion where glass particles are deposited) in order to stabilize the flow of air in the reaction chamber and thereby stabilize the burner flames and the flow of glass particles.
Describing more precisely the above apparatus for producing an optical fiber porous glass preform disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1-242431, the glass source gas is introduced into the oxyhydrogen flames, of a plurality of burners to generate glass particles by the hydrolysis reaction of the flames. These deposit on the target bar to form an optical fiber soot body. The optical fiber soot body is surrounded by the reaction chamber. In other words, a hydrolysis reaction is carried out in the reaction chamber due to the flames. By providing an exhaust outlet on one of the sides of the reaction chamber in the horizontal direction and applying a negative pressure to the exhaust outlet side, the glass particles which did not adhere to the target bar and unreacted gases are exhausted. Furthermore, on the side of the reaction chamber at the opposite side to where the exhaust outlet is provided, that is, on the other side of the reaction chamber, a fan is provided to forcibly send air toward the target bar and the exhaust outlet so as to stabilize the flow of air in the reaction chamber and thereby stabilize the burner flames and the flow of the glass particles. Both the cladding burner and the core burner are provided at the side with the fan. That is, the fan is provided on the same side as the burners.
The target bar is suspended in a direction perpendicular to the burners and the exhaust outlet, that is, vertically, and is rotated about its longitudinal center axis and pulled upward as the glass particles deposit and accumulate at the lower portion of the target bar and the optical fiber soot body grows larger. Accordingly, at the upper part of the reaction chamber, a sleeve portion for accommodating the target bar and the synthesized optical fiber soot body and a mechanism for pulling up the target bar while rotating the same are provided.
However, the apparatus for producing an optical fiber porous glass preform disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1-242431 suffers from the following disadvantages:
1. A larger size of the optical fiber soot body requires larger sizes of the upper sleeve portion and the mechanism for pulling up the target bar. Consequently, the building in which the apparatus for producing the optical fiber porous glass preform is installed has to be larger.
2. Even if a fan is provided, along with formation of the optical fiber soot body, the optical fiber soot body, formed on the target bar by the glass particles deposited thereon, is pulled upward and inserted into the upper sleeve at the upper part of the reaction chamber. Depending on how much is inserted, the flow of air in the reaction chamber changes. As a result, the flow of air cannot be stabilized as much as hoped for. Namely, the target bar is pulled upward so as to keep at a constant position the tip of the target bar where glass particles are deposited from the burner. When the optical fiber soot body is inserted gradually into the upper sleeve portion of the reaction chamber along with its formation, the flow of air in the reaction chamber changes and the position of the flames is lowered. As a result, the speed of growth (speed of formation) in the vertical direction falls and the outer diameter of the optical fiber soot body became larger than desired.
3. To prevent the above problems, it may be considered to cause the production of a forced, downward air flow from the upper sleeve, but this would collide with the rising flow of air naturally generated due to the heat and would cause turbulence, so it would be difficult to stabilize the flow of air in the reaction chamber.